Water Saturation Inversion Using Time-Lapse Seismic Data




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A formula that explicitly expresses the change of seismic reflectivity from a 4D seismic baseline survey to a monitor survey in terms of change of water saturation, effective porosity, and initial water saturation results from pressure-dependent Biot-Gassmann fluid substitution modeling and a log-derived height-saturation-porosity curve based on a Leverette-J function. Porosity and initial water saturation, which can be estimated from the baseline survey using multi-attribute analysis, are required inputs. Inversion of time-lapse seismic data for change of water saturation from the baseline survey to the monitor survey is achieved using sequential quadratic programming. Alternatively, an additional formula expressed in terms of the change of water contact level from the baseline survey to the monitor survey is solved by searching for the global minimum of error between modeled and real time-lapse seismic differences. Then free water level is indirectly obtained using a previously estimated height-saturation-porosity function. The application of the proposed inversion algorithms on synthetic models results in excellent correlation and root-mean-square error between reservoir models and inverted results. The application of the proposed inversion algorithms on a real time-lapse seismic difference volume for the Maui-B Field in the Taranaki Basin, New Zealand, shows that gas depletion and rising water level is much more significant on the southern flank. These observations agree with production logs.



4D seismic monitoring, water saturation inversion